1. Field of the Invention
The invention relates to a method for blowing combustible, optionally solids-laden gases into the free space above a molten metal located in a metallurgical vessel, in particular molten steel in an RH vessel which is under vacuum, by means of a cooled lance. The invention further relates to a blowing lance for carrying out the method.
2. Discussion of Prior Art
WO 97/08348 has disclosed a method for refining metals in a vacuum vessel, in which a lance is used having a central pipe which is surrounded by an encasing pipe which is arranged coaxially. In this case, the central pipe is in the form of a straight cylinder and extends all the way to the end of the encasing pipe. The encasing pipe itself diverges conically in its end region.
Furthermore, WO 96/16190 has disclosed a multifunctional lance which can be used for the vacuum treatment of steel in an RH vessel [vacuum vessel for removing oxygen from steel] and which allows the processes of oxygen blowing with and without solids and the generation of a combustion flame independently of one another. This multifunctional lance has a displaceable central pipe which is in the form of a straight cylinder and is arranged inside an encasing pipe, which has an end which widens conically, coaxially with respect to this encasing pipe.
Particularly during combustion, operating with these known lances causes considerable noise pollution. A further drawback is the relatively complicated design of the lance.
The invention is based on the object of providing a method and a suitable blowing lance which, by simple design means, considerably reduces the noise emissions without lowering the individual introduction rates of the media, in particular in the combustion phase.
According to the invention, the first gas flow, which is guided through the central pipe, runs in such a way that, on leaving the end of the central pipe, during re-expansion it comes into contact with the. second gas flow which surrounds it and is guided through the encasing pipe. In the process, the first gas flow is reflected by the second gas flow and/or the inner wall of the encasing pipe of the lance and, as a result, is bundled outside the lance, downstream thereof. Surprisingly, it has emerged that this reflection has scarcely any adverse effect on the output rate, yet the subsequent intensive bundling considerably reduces the noise.
The blowing lance used in this method has a cooled encasing pipe, in which a central pipe is coaxially arranged, the end of which is designed in the form of a Laval nozzle. The nozzle opening of the central pipe ends inside the encasing pipe, specifically at a distance of a=0.5 to 0.8xc3x97d, where d is the clear diameter of the central pipe.
In one embodiment, the first gas flow, which is guided through the central pipe at a pressure of 4 to 6 bar, is set in vibration at the end of the central pipe. This gas which has been set in vibration can be securely bundled even at high gas speeds. For this purpose, a chamber which serves as a vibration generator is provided in that part of the central pipe which is in the form of a Laval nozzle. This chamber is of annular design and substantially follows the inner wall of the Laval nozzle, or alternately is of entirely cylindrical design.
In an advantageous embodiment, it is proposed for the central pipe to be moved axially in a defined manner, in order to keep the noise to a minimum level according to the quantities of gas currently being blown.
Furthermore, it is proposed for the encasing pipe at its end to converge conically, at an angle xcex1 of 1 to 10xc2x0, over an area corresponding to the distance a, in the direction of gas flow. This embodiment helps to bundle together the first gas flow which emerges from the central pipe.